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SIGACTION(2)		    BSD	System Calls Manual		  SIGACTION(2)

     sigaction -- software signal facilities

     Standard C	Library	(libc, -lc)

     #include <signal.h>

     sigaction(int sig,	const struct sigaction * restrict act,
	 struct	sigaction * restrict oact);

     The system	defines	a set of signals that may be delivered to a process.
     Signal delivery resembles the occurrence of a hardware interrupt: the
     signal is blocked from further occurrence,	the current process context is
     saved, and	a new one is built.  A process may specify a handler to	which
     a signal is delivered, or specify that a signal is	to be ignored.	A
     process may also specify that a default action is to be taken by the sys-
     tem when a	signal occurs.	A signal may also be blocked, in which case
     its delivery is postponed until it	is unblocked.  The action to be	taken
     on	delivery is determined at the time of delivery.	 Normally, signal han-
     dlers execute on the current stack	of the process.	 This may be changed,
     on	a per-handler basis, so	that signals are taken on a special signal

     Signal routines execute with the signal that caused their invocation
     blocked, but other	signals	may yet	occur.	A global signal	mask defines
     the set of	signals	currently blocked from delivery	to a process.  The
     signal mask for a process is initialized from that	of its parent (nor-
     mally empty).  It may be changed with a sigprocmask(2) call, or when a
     signal is delivered to the	process.  Signal masks are represented using
     the sigset_t type;	the sigsetops(3) interface is used to modify such

     When a signal condition arises for	a process, the signal is added to a
     set of signals pending for	the process.  If the signal is not currently
     blocked by	the process then it is delivered to the	process.  Signals may
     be	delivered any time a process enters the	operating system (e.g.,	during
     a system call, page fault or trap,	or clock interrupt).  If multiple sig-
     nals are ready to be delivered at the same	time, any signals that could
     be	caused by traps	are delivered first.  Additional signals may be	pro-
     cessed at the same	time, with each	appearing to interrupt the handlers
     for the previous signals before their first instructions.	The set	of
     pending signals is	returned by the	sigpending(2) function.	 When a	caught
     signal is delivered, the current state of the process is saved, a new
     signal mask is calculated (as described below), and the signal handler is
     invoked.  The call	to the handler is arranged so that if the signal han-
     dling routine returns normally the	process	will resume execution in the
     context from before the signal's delivery.	 If the	process	wishes to re-
     sume in a different context, then it must arrange to restore the previous
     context itself.

     struct sigaction includes the following members:

	   void	     (*sa_sigaction)(int sig, siginfo_t	*info, void *ctx);
	   void	     (*sa_handler)(int sig);
	   sigset_t  sa_mask;
	   int	     sa_flags;

     When a signal is delivered	to a process a new signal mask is installed
     for the duration of the process' signal handler (or until a
     sigprocmask(2) call is made).  This mask is formed	by taking the union of
     the current signal	mask, the signal to be delivered, and the signal mask
     associated	with the handler to be invoked,	sa_mask.

     sigaction() assigns an action for a specific signal.  If act is non-zero,
     it	specifies an action (SIG_DFL, SIG_IGN, or a handler routine) and mask
     to	be used	when delivering	the specified signal.  If oact is non-zero,
     the previous handling information for the signal is returned to the user.

     Once a signal handler is installed, it remains installed until another
     sigaction() call is made, or an execve(2) is performed.  A	signal-spe-
     cific default action may be reset by setting sa_handler to	SIG_DFL.  The
     defaults are process termination, possibly	with core dump;	no action;
     stopping the process; or continuing the process.  See the signal list be-
     low for each signal's default action.  If sa_handler is set to SIG_DFL,
     the default action	for the	signal is to discard the signal, and if	a sig-
     nal is pending, the pending signal	is discarded even if the signal	is
     masked.  If sa_handler is set to SIG_IGN, current and pending instances
     of	the signal are ignored and discarded.

     Options may be specified by setting sa_flags.

     SA_NODEFER	    If set, then the signal that caused	the handler to be exe-
		    cuted is not added to the list of block signals.  Please
		    note that sa_mask takes precedence over SA_NODEFER,	so
		    that if the	specified signal is blocked in sa_mask,	then
		    SA_NODEFER will have no effect.

     SA_NOCLDSTOP   If set when	installing a catching function for the SIGCHLD
		    signal, the	SIGCHLD	signal will be generated only when a
		    child process exits, not when a child process stops.

     SA_NOCLDWAIT   If set, the	system will not	create a zombie	when the child
		    exits, but the child process will be automatically waited
		    for.  The same effect can be achieved by setting the sig-
		    nal	handler	for SIGCHLD to SIG_IGN.

     SA_ONSTACK	    If set, the	system will deliver the	signal to the process
		    on a signal	stack, specified with sigaltstack(2).

     SA_RESETHAND   If set, the	default	action will be reinstated when the
		    signal is first posted.

     SA_RESTART	    Normally, if a signal is caught during the system calls
		    listed below, the call may be forced to terminate with the
		    error EINTR, the call may return with a data transfer
		    shorter than requested, or the call	may be restarted.
		    Restarting of pending calls	is requested by	setting	the
		    SA_RESTART bit in sa_flags.	 The affected system calls in-
		    clude open(2), read(2), write(2), sendto(2), recvfrom(2),
		    sendmsg(2) and recvmsg(2) on a communications channel or a
		    slow device	(such as a terminal, but not a regular file)
		    and	during a wait(2) or ioctl(2).  However,	calls that
		    have already committed are not restarted, but instead re-
		    turn a partial success (for	example, a short read count).

		    After a fork(2) or vfork(2)	all signals, the signal	mask,
		    the	signal stack, and the restart/interrupt	flags are in-
		    herited by the child.

		    The	execve(2) system call reinstates the default action
		    for	all signals which were caught and resets all signals
		    to be caught on the	user stack.  Ignored signals remain
		    ignored; the signal	mask remains the same; signals that
		    restart pending system calls continue to do	so.

		    See	signal(7) for comprehensive list of supported signals.

     SA_SIGINFO	    If set, the	signal handler function	will receive addi-
		    tional information about the caught	signal.	 An alterna-
		    tive handler that gets passed additional arguments will be
		    called which is named sa_sigaction.	 The sig argument of
		    this handler contains the signal number that was caught.
		    The	info argument contains additional signal specific in-
		    formation which is listed in siginfo(2).  The ctx argument
		    is a pointer to the	ucontext(2) context where the signal
		    handler will return	to.

     SA_NOKERNINFO  This flag is relevant only to SIGINFO, and turns off
		    printing kernel messages on	the tty.  It is	similar	to the
		    NOKERNINFO flag in termios(4).

     Only functions that are async-signal-safe can safely be used in signal
     handlers, see signal(7) for a complete list.

     The mask specified	in act is not allowed to block SIGKILL or SIGSTOP.
     This is enforced silently by the system.

     A 0 value indicates that the call succeeded.  A -1	return value indicates
     an	error occurred and errno is set	to indicate the	reason.

     sigaction() will fail and no new signal handler will be installed if one
     of	the following occurs:

     [EFAULT]		Either act or oact points to memory that is not	a
			valid part of the process address space.

     [EINVAL]		sig is not a valid signal number.

     [EINVAL]		An attempt is made to ignore or	supply a handler for

     [EINVAL]		The sa_flags word contains bits	other than

     kill(1), kill(2), ptrace(2), sigaltstack(2), siginfo(2), sigprocmask(2),
     sigsuspend(2), setjmp(3), sigsetops(3), tty(4), signal(7)

     The sigaction() function conforms to ISO/IEC 9945-1:1990 ("POSIX.1").
     The SA_ONSTACK and	SA_RESTART flags are Berkeley extensions, available on
     most BSD-derived systems.

BSD				 June 3, 2006				   BSD


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